As discussed in Chapter 11 of Control Loop Foundation – Batch and Continuous Processes, some field devices used in the process industryrequire that the control system provide a pulsed output interface. For example, in the plastics industry extruders are often used to process plastic pellets into various products. Plastic pellets are fed into the barrel of the extruder, where they are heated using electric heater bands. As a close-fitting screw turns inside the barrel, the pellets move through the extruder and melt. To maintain the temperature of the liquid plastic at a desired setpoint value, it is necessary to adjust the energy input to the extruder. In the case of the extruder heater band, energy input is regulated by applying voltage to the heater band for a fraction of the duty cycle, and then removing the voltage for the remainder of the duty cycle. To replace the heat energy lost during extruder operation the barrel temperature is maintained at setpoint by adjusting the ON time over the duty cycle period. In the application of duty cycle control, the duty cycle time should be set at a value that is small compared to the process response time.
To provide duty cycle control, most process control systems include an interface to regulate a discrete output. For example, the discrete output card may be designed to allow a channel to be configured for duty cycle control and the AO block designed to work directly with a discrete channel. When such flexibility is provided, the AO block regulates the ON time associated with the discrete output channel. For example, if the AO block setpoint is changed to zero, then the discrete output will remain zero; it never turns on. However, if the AO block setpoint is changed to 50% then the discrete output will be turned on for 50% of the duty cycle time and
then will be turned off for the remaining portion of the duty cycle. Thus, the setup for PID control for duty cycle control is quite straight forward as illustrated below.
Duty cycle control also may be applied in the chemical industry to regulate on-off valves that are used to meter small amounts of material into a tank, and in the life sciences industry, duty cycle control is used to regulate the on-off state of peristaltic pumps that are used to provide feedstock to a bioreactor.